I. Field of the Invention
This invention relates generally to an intravascular deliverable stent or graft. In particular this invention relates to a unique stent or graft for the treatment of aneurysms, lesions, and saphenous vein grafts by reinforcing, excluding, bridging, or lining a blood vessel. Although emphasis is given to such a stent or graft specifically designed for addressing aneurysms and particularly iliac artery and abdominal aortic aneurysm (AAA), other embodiments suitable for saphenous vein graft (SVG), dialysis graft, and carotid arteries are also disclosed.
II. Discussion of the Prior Art
An aortic aneurysm is a weak area in the aorta, the main blood vessel that carries blood from the heart to the rest of the body. The aorta extends upwards from the heart in the chest and then arches downwards, traveling through the chest (the thoracic aorta) and into the abdomen (the abdominal aorta). The normal diameter of the abdominal aorta is about one inch (25 mm). As blood flows through the aorta, the weak area bulges like a balloon and can burst if the balloon gets too big.
Most commonly, aortic aneurysms occur in the portion of the vessel below the renal artery origins. The aneurysm may extend into the vessel's supplying the hips and pelvis, including the iliac arteries.
Once an aneurysm reaches 5 cm (about 2 in.) in diameter, it is usually considered necessary to treat to prevent rupture. Below 5 cm, the risk of the aneurysm rupturing is lower than the risk of conventional surgery in patients with normal surgical risks. The goal of therapy for aneurysms is to prevent them from rupturing. Once an AAA has ruptured, the chances of survival are low, with 80-90 percent of all ruptured aneurysms resulting in death. These deaths can be avoided if the aneurysm is detected and treated before it ruptures and ideally treated at an early stage (smaller aneurysm) with a lower risk procedure.
Most aortic aneurysms occur in the abdominal aorta, the main cause being arteriosclerosis. This is a condition in which fatty deposits are laid down in the walls of the arteries, which are less elastic and weaker as a result. Major risk factors for arteriosclerosis are smoking and high blood pressure as well as genetic factors.
AAA can be diagnosed from their symptoms when they occur, but this is often too late. They are usually found on routine physical examination and chest and abdominal X-rays. On examination, a doctor may feel a pulsating mass in the abdomen. If the doctor suspects an aneurysm, he/she will probably request that an ultrasound scan be carried out. Other scans, such as computerized tomography (CT) and magnetic resonance imaging (MRI) may also be performed. These scanning techniques are very useful for determining the exact position of the aneurysm.
The surgical procedure for treating AAA involves replacing the affected portion of the aorta with a synthetic graft, usually comprising a tube made out of an elastic material with properties very similar to that of a normal, healthy aorta. This major operation is usually quite successful with a mortality of between 2 and 5 percent. Even during surgery, the risk of death from a ruptured AAA is about 50%.
More recently, instead of performing open surgery in undertaking aneurysm repair, vascular surgeons have installed an endovascular stent/graft delivered to the site of the aneurysm using elongated catheters that are threaded through the patient's blood vessels. Typically, the surgeon will make a small incision in the patient's groin area and then insert a delivery catheter containing a collapsed, self-expanding or balloon-expandable stent/graft to a location bridging the aneurysm, at which point the stent/graft is delivered out from the distal end of the delivery catheter and allowed or made to expand to approximately the normal diameter of the aorta at that location. The stent/graft, of course, is a tubular structure allowing blood flow through the lumen thereof and removing pressure from the aneurysm. Over time, the stent/graft becomes endothelialized and the space between the outer wall of the stent and the aneurysm ultimate fills with clotted blood. At this time, the aneurysm is no longer subjected to aortic pressures and thus will not continue to grow.
In treating AAA, it is important that the stent or graft be accurately placed so as not to occlude blood flow through the renal arteries which branch off from the abdominal aorta.
In the Amplatz et al. U.S. Pat. No. 6,932,837, there is described a collapsible stent/graft designed for grafting a lumen of a selected blood vessel or other tubular organ. The stent/graft comprises a woven or braided fabric made from a plurality of strands of a shape memory alloy. The fabric is formed as a tube and each end of the device is open to allow fluid flow therethrough. The device can be longitudinally stretched to thereby reduce its diameter, allowing it to be inserted within the lumen of a delivery catheter. When ejected from the distal end of the delivery catheter, the stent/graft will self-expand to a predetermined outer diameter sufficient to engage the wall of the tubular vessel being treated.
While the device in the '837 patent is altogether suitable for use as a coronary stent, it is not well suited for the intravascular treatment of AAA. That device is of a uniform weave, but necessarily is of a wire density that is insufficient to limit the exposure of the aneurysm to aortic blood pressure. Should this stent/graft also encroach upon the ostia of the renal arteries, it could restrict blood flow to the kidneys.
A need, therefore, exists for a stent/graft that can be placed using an endovascular approach in the treatment of AAA, but that will not unduly occlude blood flow to the kidneys. The present invention provides such a device.
Aneurysms may also occur in the thoracic aorta where the renal arteries are not involved in the procedure, or in other arteries in the body. Depending on where the aneurysm is in relation to other branch vessels, different design variations may be needed. In some cases, where no branch vessels are involved, the stent/graft wall may be uniform throughout. In cases involving the upper aorta, one portion of the stent/graft, either the proximal, distal or area in between, may need to have a portion of the stent/graft wall with larger openings to allow adequate flow to the carotid arteries as compared to the balance of the stent/graft where the wall may have much smaller openings.
Regardless of the aneurysm site being treated, there is a need for an improved stent or vascular graft that can be collapsed to a very small deliverable diameter to reduce the arterial puncture access sheath size, trauma to the vessel at the access site and to provide for a smaller, more trackable delivery system that is less traumatic to the vasculature. There is also a need for a low profile stent or graft that provides vascular support, exclusion of aneurysms, and a surface for rapid endothelialization. The invention herein provides such benefits.
In the field of interventional cardiology, it is now becoming routine to treat stenotic lesions, in the vascular system, including saphenous vein grafts and carotid arteries, using balloon angioplasty to render more patent a partially occluded blood vessel and to attempt to thwart restenosis by placement of a stent at the site of the treated lesion.
Stents used in these procedures must be capable of assuming a reduced diameter configuration for delivery through a guide catheter or arterial sheath, but which is either self-expanding upon exit of the distal end of the guide catheter or “balloon expandable”.
In carrying out a balloon angioplasty procedure with stenting, the Seldinger technique is frequently used to gain access to the vascular system and a tubular introducer having a hemostatic valve for preventing blood loss is inserted through the puncture wound from the skin into the artery. In order to perform the procedure via percutaneous access without surgical cut down to expose the femoral artery, an introducer sheath smaller than 14 Fr (typically 6-8 Fr) is required in most patients. The smaller the introducer sheath, the less trauma to the tissue and the easier it is to place and to close the arterial puncture after the procedure. In some cases a long arterial sheath substitutes for a short vascular access sheath and provides a guiding path for delivery of devices to a site proximal the target treatment location. In other cases, a guide catheter is inserted through the introducer sheath and routed through the vascular system until the distal end portion of the guide catheter is disposed at the ostium of a selected artery having the stenotic lesion. Recently, steerable sheaths have been available for difficult to reach locations where sharp bends are encountered.
Next, a catheter may be advanced over a guide wire through the sheath or guide catheter, through the artery to the target treatment site. The catheter may be a balloon catheter, with or without a balloon expandable stent mounted over the balloon, or may be a delivery catheter for a self expanding stent. Treatment typically involves dilation of the stenotic lesion, followed by placement of a stent at the lesion site. Upon inflation of the balloon, the stenotic region of the artery having a restriction to flow is expanded in diameter to restore normal blood flow through the arterial segment. A balloon expandable or self expanding stent may next be placed in the dilated lesion site to maintain the vessel wall in the expanded diameter state. Balloon expandable stents are placed by inflating a balloon having a stent mounted thereon at the lesion site. Self expanding stents are typically placed by pulling back a sheath covering a compressed stent mounted at the distal end of the catheter. Following self expansion of the stent a balloon dilatation may optionally be used to seat the stent and ensure full expansion. Following the treatment, the catheter, guide wire, sheath, etc. are removed from the body and the vascular access site sealed by compression or other sealing means available.
Stents intended for use in percutaneous transluminal angioplasty applications come in various lengths and diameters to generally approximate the lesion length and normal range of vessel inside diameters at the various treatment sites throughout the body.
Saphenous vein graft (SVG) treatment, following a previous coronary by-pass procedure, often occurs after aging grafts become diseased and filled with a soft grumous material that can easily embolize during stenting or angioplasty. Such emboli can cause obstructions in the coronary arteries downstream to which the grafts are connected.
There is a need for a stent or graft that can be delivered in a very low profile in the collapsed state and that can line the SVG, providing vessel support, as well as preventing emboli from the SVG wall from reaching the coronary arteries.
Another recent treatment of lesions involves the carotid arteries and, in particular, treating the lesions with self expanding stents following balloon angioplasty. Since these arteries, internal carotids, lead to the brain and such lesions often contain friable plaque that can break off and cause strokes, it is necessary to deploy a distal filter or other proximal occlusion/extraction means, during the procedure to prevent emboli from reaching the brain during the procedure. Although this protection means helps to prevent emboli during the procedure, the stent itself does not provide adequate protection for preventing emboli after the stent deployment procedure due to the large open areas in the stent at the vascular wall surface.
There is a need for a stent for the carotid arteries that provides adequate vascular support as well as provides protection from emboli, and therefore stroke prevention, after the procedure is completed and the procedural protection systems are no longer in place.
In attempts to anchor prior art stents against unwanted migration following implant, vessel wall-engageable hooks are believed to cause damage to the endothelium and the situs of stenotic lesions. Thus, a need also exists for a stent or grant that is self-anchoring without requiring tissue penetrating hooks.